This invention relates to the treatment of tantalum powder for electrolytic capacitors to prevent oxygen uptake by the powder following thermal treatment in the absence of oxygen and before exposure to an oxygen-containing fluid, specifically air. The treatment involves contacting the powder with a non-aqueous solution of ammonium thiocyanate in N,N'-dimethylformamide or N-methylpyrrolidinone.
The oxygen concentration of tantalum powder destined to be used in making electrolytic capacitors is critical. When the total oxygen content of porous tantalum pellets is above approximately 4000 ppm, capacitors using such pellets as electrodes have unsatisfactory life characteristics.
Tantalum powder has a great affinity for oxygen, and the powder particles readily oxidize on exposure to air, particularly after a thermal treatment step. While technology exists to carry out all processing steps in an inert atmosphere, it is uneconomic to do so. A more viable approach is to prevent oxidation of the powder in at least one stage of its processing and thus reduce the total oxygen content in this manner.